In a planar antenna module that has a plurality of antennas formed on the same plane and carries out transmission and reception in a millimeter wave band, a third waveguide opening (65) formed in a fourth ground plate (14) and a fourth waveguide opening (66) formed in a ninth ground plate (19) are connected by a waveguide slot portion (8) formed in the ninth ground plate (19), as illustrated in FIG. 1. Such a planar antenna is disclosed for example in Japanese Patent Application Laid-open Publication No. 2002-299949.
In the planar antenna module using a prior art port-connection method illustrated in FIG. 1, when the fourth ground plate (14) and the ninth ground plate (19) illustrated in FIGS. 2(a) to 2(d) are not firmly attached on a separation portion for a waveguide slot portion (8) adjacent thereto, there will be an increased loss in a waveguide portion formed by the waveguide slot portion (8) of the ninth ground plate (19) and the fourth ground plate (14), and an electricity leak to adjacent waveguide portions. For example, when the desired frequency is in an extremely high frequency band such as a 76.5 GHz band, even if the separation portion of the waveguide slot portion (8) contacts the fourth ground plate (14) as closely-attached as possible by improving flatness of the contact surfaces, or the surface roughness of the waveguide slot portion (8) is improved as much as possible by producing the fourth ground plate (14) and the ninth ground plate (19) from a cutting work product, a loss of about 0.3 dB per unit length of 1 cm is inevitable. Since a waveguide that connects an input/output port of the antennas, that is, a third waveguide opening (65) formed in the fourth ground plate (14), and an input/output port of a millimeter wave circuit, that is, a fourth waveguide opening (66) formed in the ninth ground plate (19), needs to be up to 5 cm long, the insertion loss taking place over the length from the input/output port of the antennas to the input/output port of the millimeter wave circuit amounts to about 1.8 dB as a whole as illustrated in FIG. 3. In addition, when the fourth ground plate (14) and the ninth ground plate (19) are made by casting or the like with the aim of reduced costs, they can be warped and undulated. As a result, a contact accuracy between the separation portion of the waveguide slot (8) and the fourth ground plate (14) is not retained and the surface protection treatment or the like is required in order to prevent corrosion. Therefore, there exists a disadvantage in that the insertion loss becomes larger when using a casting method than when using a cutting work product to make the ground plates (14) (19) and thus cost reduction becomes difficult.
In a planar array antenna for use in an in-vehicle radar or high speed communications in a millimeter wave band, it is important to realize a high gain and wide band characteristic. The inventors of the present invention have configured an antenna illustrated in FIG. 11 as a high-gain planar antenna applicable to such a usage in order to examine a reduction in feeder loss and undesired feeder radiation (See Japanese Patent Application Laid-open Publication No. H04-082405).
In such an antenna, a traverse component of energy propagating in a traverse direction is generated between the ground plate and the slot plate, except for an energy component radiated directly outward from the slot, when the patch is excited via the feeder. It has been known that the traverse component is then radiated out from the adjacent slot, thereby placing an adverse effect on an array-antenna gain, the effect being caused due to a phase relation with the component radiated directly outward from the slot. Namely, the maximum in the array-antenna gain appears at a particular arrangement distance as illustrated in FIG. 13, thereby realizing a high gain and highly efficient antenna.
In addition, in such usages, in order to detect a direction of a vehicle ahead or automatically choose a direction that yields a high sensitivity, a transmitting antenna and a plurality of receiving antennas are integrally constructed as illustrated in FIG. 14 and a signal received by each antenna can be subjected to a phase control and a selective synthesis, thereby enabling a beam direction control and a selective extraction of the signal coming from a particular direction.
In this case, since detection accuracy for a particular direction and a detection range can be improved by making uniform a gain and directivity of a plurality of the receiving antennas, it is important to realize uniform characteristics over the receiving antennas.
As described above, in case of the triple plate planar antenna constructed integrally with the transmitting antenna and the plurality of the receiving antennas, it is difficult to make uniform the antenna gain and directivity, since a component of energy propagating in a traverse direction is different in a center portion of the antenna array from in a peripheral portion of the antenna array. Although it is considered to provide a parasite element electromagnetically-coupled to a radiation element as illustrated in FIG. 12 to reduce a component of energy propagating in a traverse direction, it is difficult to address it due to an increase of the number of elements etc.
By the way, in recent years, an adoption of the system in which a feeder is configured into a triple plate type has become a main stream in a planar antenna in a microwave and millimeter wave band (See Japanese Utility Model Application Laid-open Publication No. H06-070305, and Japanese Patent Application Laid-open Publication No. 2004-215050, for example). In the planar antenna adopting the triple plate feeder system, electrical power to be fed with each antenna element is synthesized by the triple plate feeder. In a connection portion of the synthesized electricity between a final output portion and an RF signal process circuit, a triple plate feeder-waveguide converter is used frequently, because it is easily assembled and has a high reliability. A structure of the conventional triple plate feeder-waveguide converter is illustrated in FIGS. 23(a) to 23(c). In this structure, in order to facilitate a conversion to the waveguide with low loss, a film substrate 140 on which a strip feeder conductor 130 is formed is arranged over the surface of the ground plate 111 via a dielectric 120a {FIGS. 23(b) and 23(c)} and an upper ground plate 150 is arranged thereabove via dielectric 120b {FIGS. 23(b) and 23(c)} so as to configure the triple plate feeder. In the following, reference is made to FIGS. 23(a)-23(c). In addition, when connecting a waveguide input portion 160 (see FIGS. 23(b) and 23(c)) of the circuit system, a through hole having the same inner dimension as that of the waveguide is provided in the ground plate 111; a metal spacer portion 170a {FIGS. 23(b) and 23(c)} having the same thickness as the dielectric 120a is provided in order to support the film substrate 140; the film substrate 140 is sandwiched by the metal spacer portion 170a and a metal spacer portion 170b {FIGS. 23(b) and 23(c)} having the same dimension; an upper ground plate 150 having a through hole with the same inner dimension as the waveguide is arranged on top of the metal spacer portion 170b in such a way that the through hole formed in the ground plate 111, a waveguide portion formed by the inner wall of the metal spacers 170a, 170b, and the through hole formed in the upper ground plate 150 coincide with one another; and a short-circuit metal plate 180 is arranged so as to close the through hole formed in the ground plate 5. An insertion length A of the strip feeder conductor 130 that is inserted into the waveguide illustrated in FIG. 23(a) and a short-circuit distance L illustrated in FIG. 23(b) are set as desired, thereby realizing the triple plate feeder-waveguide converter having a low loss in a wider frequency band intended to be utilized.
In the conventional triple plate feeder-waveguide converter illustrated in FIGS. 23(a) to 23(c), since a wavelength of electromagnetic wave in a millimeter wave band, for example, an electromagnetic wave having a frequency of about 76 GHz, is short, only a slight degradation in mechanical accuracy of the insertion length A of the strip feeder conductor (130 (FIGS. 23(a) and 23(b)) and the short-circuit length L (FIG. 23(b) can lead to a deterioration in reflection characteristics. Therefore, a machining method realizing a high mechanical accuracy or an adoption of a structure yielding a high precision is prerequisite. Additionally, in order to adjust the short-circuit length L, a short-circuit length adjustment metal plate 190 (FIGS. 23(c) and 24(d)) having a through hole with an inner dimension that is the same as that of the waveguide may be required, as shown in FIG. 23(c). Therefore, there exits a disadvantage in that a production cost is raised by an increased number of parts.
The objective of the present invention is an inexpensive provision of a planar antenna module that is able to realize a reduction in loss, a reduction in characteristic variation caused by an assembling error, and an improved stability in frequency characteristics.
Another objective of the present invention is a provision of a triple plate planar array antenna that is able to realize a uniform antenna characteristic between antennas in the center portion and those in the peripheral portion of the antenna array configured by arranging a plurality of compact-sized antennas therein.
Yet another objective of the present invention is an inexpensive provision of an easy-to-assemble triple plate feeder-waveguide converter that is able to make unnecessary the short-circuit metal plate 180 and the short-circuit length adjustment metal plate 190, both of which have been required in a conventional structure, without impairing a low loss characteristic that has been conventionally realized, and that has a high connection reliability.